The expression “active layer” is understood to mean a layer (or a plurality of sublayers) on which or in which components intended especially for applications in the fields of microelectronics, optics, optoelectronics, piezoelectricity or even spintronics will be fabricated.
The expression “strain state” is understood to mean the strains resulting from internal stresses between deformed portions of the useful layer, the internal stresses possibly being tensile or compressive stresses. When the internal stresses are zero or almost zero, a “relaxed state” is spoken of to designate the corresponding strain state.
An active layer made of a semiconductor is placed under (tensile or compressive) strain especially in order to modify its electronic band structure. This has the consequence of modifying its electron transport properties or its electromagnetic properties. From the electronic point of view, carrier mobility may be improved. From the electromagnetic point of view, the modification of the strain state leads to a modification of the valence and conduction bands and possibly of the (direct or indirect) bandgap of semiconductors and insulators.
To improve component performance, an active layer with a high strain level is required, meaning that an active layer must be able to undergo a substantial deformation (or in other words, a substantial relative extension). Thus, more precisely, it is sought to obtain an active layer that can be deformed by more than 0.75%, or even more than 1%, without creating defects.
The document entitled “Fabricating Strained Silicon Substrates Using Mechanical Deformation during Wafer Bonding,” K. T. Turner, ECS Transactions, 16(8) 321-328 (2008), denoted D1 below, discloses (see FIG. 2 especially) a process for modifying an initial strain state of an active layer to a final strain state, the process comprising steps of a) providing a first substrate comprising the active layer in the initial strain state, the active layer being made of a first material having a Young's modulus denoted E1, the active layer having a thickness denoted h1; b) providing a second substrate made of a second material having a Young's modulus denoted E2, the second substrate having a thickness denoted h2, the second substrate having an initial shape at rest; c) bending the first substrate and the second substrate so that they each have a curved shape of substantially identical radius of curvature denoted R; d) joining the second substrate to the active layer so that the second substrate closely follows the shape of the first substrate; and e) re-establishing the initial at-rest shape of the second substrate so that the active layer has the final strain state.
D1 discloses (compare FIGS. 4(a) and 4(b) and see the first paragraph on page 327) that it is preferable to use a second substrate with a second material regarding the relationship E2=E1 (i.e., E2/E1=1/Σ=1 following the notations of D1) for a given thickness ratio h2/h1 (h2/h1=1/ξ=103). D1 teaches that such a second substrate, in conjugation with the decrease in the thickness of the first substrate after the joining step d), allows the active layer to be substantially deformed and, therefore, high strains to be obtained therein. A contrario, D1 teaches that the flexibility of the second substrate (Σ=100) relative to the first substrate, in conjugation with the decrease in the thickness of the first substrate after the joining step d), only leads to 70% of the value of the strains obtained for Σ=1, for the same given thickness ratio h2/h1, (h2/h1=1/ξ=103).
The process described in D1 is not entirely satisfactory when the first substrate has a substantial stiffness; for example, if the first substrate is made of a first semiconductor, such as silicon, then the second substrate also has a substantial stiffness so as to relate to Σ=1. Such a second substrate may then be difficult to bend in step c) without creating defects if it is desired to deform the active layer by more than 0.75%, this especially being the case when the second substrate is of substantial size (for example, h2/h1=1/ξ=103).